Overvoltage Protection Circuit, LED Backlight Driving Circuit and LCD

ABSTRACT

An OVP circuit includes a voltage boost circuit, for boosting an input voltage into an output voltage in need and providing the output voltage to a load, a voltage control module for controlling the voltage boost circuit to boost an input voltage into an output voltage in need and for providing the output voltage to the load, an overvoltage protection module for monitoring a voltage of a positive terminal of the load to enable or disable the voltage control module, and an overvoltage adjusting module for monitoring an operation voltage of the load to adjust the overvoltage. It effectively prevents from the abnormal operation attributed by over practical operation voltage or prevents components from damage attributed by tardy protection. The present invention also proposes an LED backlight driving circuit using the OVP circuit and an LCD having the LED backlight driving circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an overvoltage protection circuit, anLED backlight driving circuit comprising the overvoltage protectioncircuit, and a liquid crystal display (LCD) having the LED backlightdriving circuit.

2. Description of the Prior Art

With technical development, a backlight technology of a liquid crystaldisplay has been developing. A conventional liquid crystal display usescold cathode fluorescence lamp (CCFL) as a backlight source. A backlightsource technology for an LED backlight source has been invented toovercome disadvantages of CCFL backlight sources, such as worse colorrestoration, low luminous efficiency, high discharge voltage, lowdischarge property in low temperature, long heating time for stablegrayscale, etc. In a liquid crystal display, an LED backlight source anda liquid crystal display panel are set up in opposite for the LEDbacklight source to provide light to the liquid crystal display panel.The LED backlight source includes at least one LED string comprising aplurality of LEDs in serial. In a process of producing or assembling LEDbacklight sources, voltage applied on the LED string is over or underpredetermined value on account of technical difference.

FIG. 1 is a circuit diagram of a driving circuit of an LED backlightsource in a conventional LCD. As FIG. 1 shows, the driving circuit ofthe LED backlight source includes a voltage boost circuit 1, an LEDstring 21, a reference voltage module 41 and a voltage control module 3.The voltage control module 3 is coupled to a reference voltage VFBsupplied by the reference voltage module 41, and the voltage controlmodule 3 controls the voltage boost circuit 1 to boost an input voltageto a needed output voltage and to supply the needed output voltage tothe LED string 21. The reference voltage module 41 comprises a resistorR1 and a resistor R2 connected in serial. The resistor R1 coupledbetween the voltage boost circuit 1 and the resistor R2. The resistor R2is grounded. The reference voltage VFB is coupled to the resistor R1 andthe resistor R2. In the circuit, a constant voltage fed to a positiveend of the LED string 21 through the reference voltage module 41 is

${{VFB}*\frac{R\; 1}{R\; 2}} + {{VFB}.}$

That is the circuit has an overvoltage protection (OVP) function with anOVP voltage value of

${{VFB}*\frac{R\; 1}{R\; 2}} + {{VFB}.}$

Therefore, if the circuit is malfunction to make the actual voltageapplied on the positive end of the LED string 21 exceeding the voltageneeded, the circuit disables because the voltage fed to the positive endof the LED string 21 is

${{VFB}*\frac{R\; 1}{R\; 2}} + {{VFB}.}$

However, the OVP voltage is incapable of adapting practical operationvoltage of the LED string 21 in need when the rated voltage of the LEDstring 21 changes significantly.

For instance, the required operation voltage of the LED string isprobably greater than the OVP voltage if the practical operation voltageof the LED string 21 in need is over to a predetermined voltage, whichmeans the LED string needs greater voltage to work normally. It occursthat the LED string does not lighten. On the contrary, the requiredoperation voltage of the LED string is probably less than the OVPvoltage if the practical operation voltage of the LED string 21 in needis under to predetermined voltage, which means the LED string needs lessvoltage to work normally. If abnormal situation happens, it takes a longtime to boost a voltage applied on the positive end of the LED to theOVP voltage so that it damages components.

SUMMARY OF THE INVENTION

Owing to defects of the prior art, the present invention proposes anovervoltage protection circuit and an LED backlight driving circuitcomprising the overvoltage protection circuit, for automaticallyadjusting OVP voltages according to a voltage which an LED string needsto prevent components from damage attributed by tardy protection orabnormal operation on account of tremendous voltage change in the LEDstring.

According to the present invention, an overvoltage protection circuitcomprises: a voltage boost circuit, for boosting an input voltage intoan output voltage in need and providing the output voltage to a load; avoltage control module, for controlling the voltage boost circuit toboost an input voltage into an output voltage in need and for providingthe output voltage to the load and to drive the load in a constantcurrent; an overvoltage protection module, for monitoring a voltage of apositive terminal of the load and generating a control signal based onthe voltage of the positive terminal of the load and a predeterminedovervoltage, the control signal being used to control the voltagecontrol module to enable or disable; and

an overvoltage adjusting module, for monitoring an operation voltage ofthe load to generate an adjustment signal according to the operationvoltage, the adjustment signal being used for adjusting an overvoltagein the overvoltage protection module.

In one aspect of the present invention, the overvoltage protectionmodule generates a first control signal to control the voltage controlmodule to enable if the overvoltage protection module monitors that thevoltage of the positive terminal of the load is under the overvoltage ofthe overvoltage protection module, and the overvoltage protection modulegenerates a second control signal to control the voltage control moduleto disable if the overvoltage protection module monitors that thevoltage of the positive terminal of the load is over the overvoltage ofthe overvoltage protection module, wherein if the overvoltage adjustingmodule monitors the operation voltage in the load is under a standardvalue, the overvoltage adjusting module generates a first adjustmentsignal, and the overvoltage protection module decreases the overvoltageaccording to the first adjustment signal, and if the overvoltageadjusting module monitors the operation voltage in the LED string isover the standard value, the overvoltage adjusting module generates asecond adjustment signal, and the overvoltage protection module augmentsovervoltage according to the second adjustment signal.

According to the present invention, a light emitting diode (LED)backlight driving circuit comprises: a voltage boost circuit, forboosting an input voltage into an output voltage in need and providingthe output voltage to an LED string; a voltage control module, forcontrolling the voltage boost circuit to boost an input voltage into anoutput voltage in need and providing the output voltage to an LED stringand to drive the LED string in a constant current; an overvoltageprotection module, for monitoring a voltage of a positive terminal ofthe LED string and for generating a control signal based on the voltageof the positive terminal of the LED string and a predeterminedovervoltage, the control signal being used to control the voltagecontrol module to enable or disable; and an overvoltage adjustingmodule, for monitoring an operation voltage of the LED string togenerate an adjustment signal according to the operation voltage, theadjustment signal being used for adjusting an overvoltage in theovervoltage protection module.

In one aspect of the present invention, the overvoltage protectionmodule generates a first control signal to control the voltage controlmodule to enable if the overvoltage protection module monitors that thevoltage of the positive terminal in the LED string is under theovervoltage of the overvoltage protection module, and the overvoltageprotection module generates a second control signal to control thevoltage control module to disable if the overvoltage protection modulemonitors that the voltage of the positive terminal in the LED string isover the overvoltage of the overvoltage protection module.

In yet another aspect of the present invention, if the overvoltageadjusting module monitors the operation voltage in the LED string isunder a standard value, the overvoltage adjusting module generates afirst adjustment signal, and the overvoltage protection module decreasesthe overvoltage according to the first adjustment signal, and if theovervoltage adjusting module monitors the operation voltage in the LEDstring is over the standard value, the overvoltage adjusting modulegenerates a second adjustment signal, and the overvoltage protectionmodule augments overvoltage according to the second adjustment signal.

In another aspect of the present invention, the overvoltage protectionmodule comprises an adjustment circuit for adjusting overvoltageaccording to an adjustment signal from the overvoltage adjusting module,and a protection circuit for monitoring and comparing a voltage of thepositive terminal of the LED string with the overvoltage to generate acontrol signal coupling to the voltage control module.

In another aspect of the present invention, the adjustment circuitcomprises a first voltage regulator, a second voltage regulator, a thirdvoltage regulator, a first field-effect transistor, a secondfield-effect transistor, a third field-effect transistor and a thirdresistor;

wherein the first voltage regulator, the second voltage regulator andthe third voltage regulator are in electrical series, a cathode of thefirst voltage regulator is coupled to the protection circuit, an anodeof the third voltage regulator is grounded;

gates of the first and the second field-effect transistors arerespectively coupled to the overvoltage adjusting module, the adjustmentsignal from the overvoltage adjusting module controls the firstfield-effect transistor and the second field-effect transistor to turnon or off, a drain of the first field-effect transistor is coupled to acathode of the first voltage regulator, a drain of the secondfield-effect transistor is coupled to a gate of the third field-effecttransistor and then coupled to a third reference voltage through thethird resistor, a drain of the third field-effect transistor is coupledto an anode of the second voltage regulator, sources of the first, thesecond and the third field-effect transistors are all grounded; and

the protection circuit comprises a fourth resistor and a fourthfield-effect transistor, one end of the fourth resistor is coupled to apositive end of the LED string, the other end of the fourth resistor iscoupled to a gate of the fourth field-effect transistor and then coupledto the cathode of the first voltage regulator, a drain of the fourthfield-effect transistor outputs a control signal coupling to the voltagecontrol module, and a source of the fourth field-effect transistor isgrounded.

In another aspect of the present invention, the overvoltage adjustingmodule comprises a divider circuit, for monitoring an operation voltageof the LED string and generating a dividing voltage, and a comparisoncircuit, for generating an adjustment signal coupling to the overvoltageprotection module based on the dividing voltage.

In another aspect of the present invention, the comparison circuitcomprises a first comparator and a second comparator, and

wherein an out-of-phase input terminal of the first comparator receivesa first reference voltage, an in-phase input terminal of the secondcomparator receives a second reference voltage, an in-phase inputterminal of the first comparator and an out-of-phase input terminal ofthe second comparator respectively receive the dividing voltage from thedivider circuit, and output terminals of the first and the secondcomparators respectively couple adjustment signals to the overvoltageprotection module, wherein the first reference voltage is over thesecond reference voltage.

In still another aspect of the present invention, the divider circuitcomprises a first resistor and a second resistor, wherein one end of thefirst resistor is coupled to the positive terminal of the LED string,the other end of the first resistor is coupled to one end of the secondresistor and then coupled to the comparison circuit, and the other endof the second resistor is grounded.

In yet another aspect of the present invention, the divider circuitcomprises a first resistor and a second resistor, wherein one end of thefirst resistor is coupled to the positive terminal of the LED string,the other end of the first resistor is coupled to one end of the secondresistor and then coupled to the comparison circuit, and the other endof the second resistor is grounded.

According to the present invention, a liquid crystal display comprisinga light emitting diode (LED) backlight source driven by an LED backlightdriving circuit is provided. The LED backlight driving circuitcomprises: a voltage boost circuit, for boosting an input voltage intoan output voltage in need and providing the output voltage to an LEDstring; a voltage control module, for controlling the voltage boostcircuit to boost an input voltage into an output voltage in need andproviding the output voltage to an LED string and to drive the LEDstring in a constant current; an overvoltage protection module, formonitoring a voltage of a positive terminal of the LED string and forgenerating a control signal based on the voltage of the positiveterminal of the LED string and a predetermined overvoltage, the controlsignal being used to control the voltage control module to enable ordisable; and an overvoltage adjusting module, for monitoring anoperation voltage of the LED string to generate an adjustment signalaccording to the operation voltage, the adjustment signal being used foradjusting an overvoltage in the overvoltage protection module.

In one aspect of the present invention, the overvoltage protectionmodule generates a first control signal to control the voltage controlmodule to enable if the overvoltage protection module monitors that thevoltage of the positive terminal in the LED string is under theovervoltage of the overvoltage protection module, and the overvoltageprotection module generates a second control signal to control thevoltage control module to disable if the overvoltage protection modulemonitors that the voltage of the positive terminal in the LED string isover the overvoltage of the overvoltage protection module.

In yet another aspect of the present invention, if the overvoltageadjusting module monitors the operation voltage in the LED string isunder a standard value, the overvoltage adjusting module generates afirst adjustment signal, and the overvoltage protection module decreasesthe overvoltage according to the first adjustment signal, and if theovervoltage adjusting module monitors the operation voltage in the LEDstring is over the standard value, the overvoltage adjusting modulegenerates a second adjustment signal, and the overvoltage protectionmodule augments overvoltage according to the second adjustment signal.

In another aspect of the present invention, the overvoltage protectionmodule comprises an adjustment circuit for adjusting overvoltageaccording to an adjustment signal from the overvoltage adjusting module,and a protection circuit for monitoring and comparing a voltage of thepositive terminal of the LED string with the overvoltage to generate acontrol signal coupling to the voltage control module.

In another aspect of the present invention, the adjustment circuitcomprises a first voltage regulator, a second voltage regulator, a thirdvoltage regulator, a first field-effect transistor, a secondfield-effect transistor, a third field-effect transistor and a thirdresistor;

wherein the first voltage regulator, the second voltage regulator andthe third voltage regulator are in electrical series, a cathode of thefirst voltage regulator is coupled to the protection circuit, an anodeof the third voltage regulator is grounded;

gates of the first and the second field-effect transistors arerespectively coupled to the overvoltage adjusting module, the adjustmentsignal from the overvoltage adjusting module controls the firstfield-effect transistor and the second field-effect transistor to turnon or off, a drain of the first field-effect transistor is coupled to acathode of the first voltage regulator, a drain of the secondfield-effect transistor is coupled to a gate of the third field-effecttransistor and then coupled to a third reference voltage through thethird resistor, a drain of the third field-effect transistor is coupledto an anode of the second voltage regulator, sources of the first, thesecond and the third field-effect transistors are all grounded; and

the protection circuit comprises a fourth resistor and a fourthfield-effect transistor, one end of the fourth resistor is coupled to apositive end of the LED string, the other end of the fourth resistor iscoupled to a gate of the fourth field-effect transistor and then coupledto the cathode of the first voltage regulator, a drain of the fourthfield-effect transistor outputs a control signal coupling to the voltagecontrol module, and a source of the fourth field-effect transistor isgrounded.

In another aspect of the present invention, the overvoltage adjustingmodule comprises a divider circuit, for monitoring an operation voltageof the LED string and generating a dividing voltage, and a comparisoncircuit, for generating an adjustment signal coupling to the overvoltageprotection module based on the dividing voltage.

In another aspect of the present invention, the comparison circuitcomprises a first comparator and a second comparator, and

wherein an out-of-phase input terminal of the first comparator receivesa first reference voltage, an in-phase input terminal of the secondcomparator receives a second reference voltage, an in-phase inputterminal of the first comparator and an out-of-phase input terminal ofthe second comparator respectively receive the dividing voltage from thedivider circuit, and output terminals of the first and the secondcomparators respectively couple adjustment signals to the overvoltageprotection module, wherein the first reference voltage is over thesecond reference voltage.

In still another aspect of the present invention, the divider circuitcomprises a first resistor and a second resistor, wherein one end of thefirst resistor is coupled to the positive terminal of the LED string,the other end of the first resistor is coupled to one end of the secondresistor and then coupled to the comparison circuit, and the other endof the second resistor is grounded.

In yet another aspect of the present invention, the divider circuitcomprises a first resistor and a second resistor, wherein one end of thefirst resistor is coupled to the positive terminal of the LED string,the other end of the first resistor is coupled to one end of the secondresistor and then coupled to the comparison circuit, and the other endof the second resistor is grounded.

The benefit of the present invention is that the overvoltage protectioncircuit is capable of automatically adjusting an overvoltage valueaccording to an operation voltage of a load so that it preventscomponents from damage attributed by tardy protection on account oftremendous voltage change applied on the load. Specifically, the LEDbacklight driving circuit comprising the overvoltage protection circuitis capable of monitoring the operation voltage in the LED string andcontrolling the OVP voltage value based on the operation voltage. Theovervoltage protection module decreases the OVP voltage value throughthe first adjustment signal from the overvoltage adjusting module if theoperation voltage in the LED string is under a standard value. Theovervoltage protection module increases the OVP voltage value throughthe second adjustment signal from the overvoltage adjusting module ifthe operation voltage in the LED string is over the standard value. Inthis way, it automatically adjusts the OVP voltage value based on thepractical operation voltage in the LED string if there is a tremendousvoltage change in the LED string. It effectively prevents from thenon-lightening error attributed by over practical operation voltage inthe LED string with lower OVP voltage, or prevents components fromdamage attributed by taking a long time to boost a voltage applied on apositive end of the LED to the OVP voltage because of lower practicaloperation voltage in the LED string with over OVP voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a driving circuit of an LED backlightsource in a conventional LCD.

FIG. 2 is a block diagram of an overvoltage protection circuit accordingto one embodiment of the present invention.

FIG. 3 shows a block diagram of an LED backlight driving circuitaccording to another embodiment of the present invention.

FIG. 4 is a circuit diagram of the LED backlight driving circuitaccording to an embodiment of the present invention.

FIG. 5 is a block diagram of the LED backlight driving circuit accordingto an embodiment of the present invention.

FIG. 6 is a circuit diagram of the overvoltage protection moduleaccording to an embodiment of the present invention.

FIG. 7 is a circuit diagram of the overvoltage adjusting moduleaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is described in detail in conjunction with theaccompanying drawings and embodiments.

FIG. 2 is a block diagram of an overvoltage protection circuit accordingto one embodiment of the present invention.

Referring to FIG. 2, the overvoltage protection circuit of theembodiment comprises a voltage boost circuit 1 for boosting an inputvoltage Vin into an output voltage Vout in need and supplying the outputvoltage Vout to a load 2, a voltage control module 3 for controlling thevoltage boost circuit 1 to boost an input voltage Vin into an outputvoltage Vout in need and for providing the output voltage Vout to theload 2 to drive the load 2 under a constant current, an overvoltageprotection module 4 for monitoring the voltage applied on the positiveend of the load 2 and generating a control signal which is used tocontrol the voltage control module 3 to enable or disable based on thevoltage applied on the positive end and a predetermined overvoltage, andan overvoltage adjusting module 5 for monitoring the operation voltageof the load 2 and generating an adjustment signal which is used to limitthe overvoltage in the overvoltage protection module 4, according to theoperation voltage to adjust the overvoltage.

The overvoltage protection module 4 generates a first control signal tocontrol the voltage control module 3 to work if the overvoltageprotection module 4 monitors that the voltage of the positive end of theload 2 is under the overvoltage of the overvoltage protection module 4.On the other hand, the overvoltage protection module 4 generates thesecond control signal to control the voltage control module 3 to stopworking if the overvoltage protection module 4 monitors that the voltageof the positive end of the load 2 is over the overvoltage of theovervoltage protection module 4 so that it practices overvoltageprotection.

If the overvoltage adjusting module 5 monitors the operation voltage ofthe load 2 is under the standard value, the overvoltage adjusting module5 generates the first adjustment signal coupling to the overvoltageprotection module 4, and the overvoltage protection module 4 decreasesovervoltage according to the first adjustment signal. On the other hand,if the overvoltage adjusting module 5 monitors the operation voltage ofthe load 2 is over the standard value, the overvoltage adjusting module5 generates the second adjustment signal coupling to the overvoltageprotection module 4, and the overvoltage protection module 4 augmentsovervoltage according to the second adjustment signal.

The overvoltage protection circuit of the embodiment is capable ofautomatically adjusting an overvoltage according to an operation voltageof a load so that it prevents components from damage attributed by tardyprotection or abnormal operation on account of tremendous voltage changeapplied on the load.

The overvoltage protection circuit is applied to the LED backlightdriving circuit as shown in FIG. 3. The load in the overvoltageprotection circuit is the LED string 21.

FIG. 4 and FIG. 5 are respectively a circuit diagram and a block diagramof the LED backlight driving circuit according to an embodiment of thepresent invention.

Refer to FIG. 4 to FIG. 7. The LED backlight driving circuit of theembodiment specifically comprises the voltage boost circuit 1, the LEDstring 21, the voltage control module 3, the overvoltage protectionmodule 4 and the overvoltage adjusting module 5.

The voltage boost circuit 1 comprises an inductor 111, a rectifyingdiode 112, a fifth field-effect transistor 113, a fifth resistor 114 anda capacitor 115. One end of the inductor 111 receives the input directcurrent (DC) voltage Vin, and the other end is coupled to an anode ofthe rectifying diode 112 and a drain of the fifth field-effecttransistor 113. A gate of the fifth field-effect transistor 113 iscoupled to the voltage control module 3, and the signal from the voltagecontrol module 3 controls the fifth field-effect transistor 113 to turnon or off. A source of the fifth field-effect transistor 113 is groundedvia the fifth resistor 114. A negative end of the rectifying diode 112is grounded through the capacitor 115 and serves as the output terminalof the voltage boost circuit 1 coupled to the LED string 21.

The voltage control module 3 comprises a control chip U1, a sixthfield-effect transistor 311, a sixth resistor 312, a seventh resistor313 and an eighth resistor 314. A drain of the sixth field-effecttransistor 311 is coupled to a negative terminal of the LED string 21,and a source of the sixth field-effect transistor 311 is electricallygrounded through the sixth resistor 312. The control chip U1 coupled tothe source of the sixth field-effect transistor 311 through a pin S1 isused for monitoring a voltage applied on the sixth resistor 312. Thecontrol chip U1 coupled to the gate of the sixth field-effect transistor311 through a pin G1 is used for controlling the sixth field-effecttransistor 311 to turn on or off. A pin ISEN of the control chip U1coupled to the source of the fifth field-effect transistor 113 in thevoltage boost circuit 1 through the seventh resistor 313 is used fordetecting current flowing through the source of the fifth field-effecttransistor 113. A pin GATE of the control chip U1 coupled to the gate ofthe fifth field-effect transistor 113 through the eighth resistor 314generates control signals to control the fifth field-effect transistor113 to turn on or off. An enable signal pin EN of the control chip U1 iscoupled to the overvoltage protection module 4 and receives the controlsignal EN from the overvoltage protection module 4. The control chip U1enables or disables according to the control signal coupled to theovervoltage protection module 4. In the meantime, the control chip U1controls the field-effect transistor 113 to turn on or off to controlthe voltage boost circuit 1 through the pin GATE by monitoring changesfor voltages of the sixth resistor 312 and current of the source of thefifth field-effect transistor 113 when the control chip U1 operatesnormally. Therefore, the voltage boost circuit 1 boosts the inputvoltage Vin into the needed output voltage Vout for the LED string 21 todrive the LED string 21 in a constant current.

As described above, the overvoltage protection module 4 monitors thevoltage of the positive terminal of the LED string 21 and generates acontrol signal EN, for controlling the voltage control module 3 toenable or disable, according to the voltage of the positive terminal andthe predetermined overvoltage. The detailed circuit diagram of theovervoltage protection module 4 is shown in FIG. 6. The overvoltageprotection module 4 comprises a protection circuit 41 and an adjustmentcircuit 42. The protection circuit 41 includes a fourth resistor 41 anda fourth field-effect transistor 412. The adjustment circuit 42 includesa first voltage regulator 421, a second voltage regulator 422, a thirdvoltage regulator 423, a first field-effect transistor 424, a secondfield-effect transistor 425, a third field-effect transistor 426 and athird resistor 427. The first voltage regulator 421, the second voltageregulator 422 and the third voltage regulator 423 are in electricalseries. The cathode of the first voltage regulator 421 is not onlycoupled to the positive terminal of the LED string 21 (i.e. the outputof the voltage boost circuit 1) through the fourth resistor 411 in theprotection circuit 41, but also coupled to the gate of the fourthfield-effect transistor 412 in the protection circuit 41 for controllingthe fourth field-effect transistor 412 to turn on or off, so that thedrain of the fourth field-effect transistor 412 outputs the controlsignal EN, for controlling the voltage control module 3 to enable ordisable. The control signal EN is at low level when the source of thefourth field-effect transistor 412 turns on, and on the contrary, thecontrol signal EN is at high level when the source of the fourthfield-effect transistor 412 turns off. The gate of the firstfield-effect transistor 424 and the gate of the second field-effecttransistor 425 are respectively coupled to the overvoltage adjustingmodule 5. The adjustment signal from the overvoltage adjusting module 5controls the first field-effect transistor 424 and the secondfield-effect transistor 425 to turn on or off. The drain of the firstfield-effect transistor 424 is coupled to the first voltage regulator421. The drain of the second field-effect transistor 425 and the gate ofthe third field-effect transistor 426 are coupled with each other andthen coupled to the third reference voltage via the third resistor 427,and controls the third field-effect transistor 426 to turn on or off.The drain of the third field-effect transistor 426 is coupled to theanode of the second voltage regulator 422. The sources of the firstfield-effect transistor 424, of the second field-effect transistor 425and of the third field-effect transistor 426 are electrically connectedto the anode of the third voltage regulator 423. In the embodiment, theovervoltage protection module 4 arranges the OVP voltage according tosets of regulating voltages of the first voltage regulator 421, thesecond voltage regulator 422 and the third voltage regulator 423.

As mentioned above, the overvoltage adjusting module 5 monitors theoperation voltage of the LED string 21 and generates an adjustmentsignal, for adjusting the overvoltage of the overvoltage protectionmodule 4, according to the operation voltage. The detailed circuitdiagram of the overvoltage adjusting module 5 is illustrated in FIG. 7.The overvoltage adjusting module 5 comprises a comparison circuit 51 anda divider circuit 52. The comparison circuit 51 comprises a firstcomparator 511 and the second comparator 512. The divider circuit 52comprises a first resistor 521 and a second resistor 522. The firstresistor 521 and the second resistor 522 are connected in series andgrounded, and the other end of the first resistor 521 is coupled to thepositive end. A dividing voltage Vfb between the first resistor 521 andthe second resistor 522 is output and coupled to an in-phase inputterminal of the first comparator 511 and an out-of-phase input terminalof the second comparator 512 in the comparison circuit 51. Theout-of-phase input terminal of the first comparison 511 receives thefirst reference voltage Vref1, and the in-phase terminal of the secondcomparator 512 receives the second reference voltage Vref2. The outputterminal of the first comparator 511 is coupled to the gate of thesecond filed-effect transistor 425 in the overvoltage protection module4. The adjustment signal Sn1 output from the first comparator 511determines the second field-effect transistor 425 to turn on or off. Theoutput terminal of the second comparator 512 is coupled to the gate ofthe first filed-effect transistor 424 in the overvoltage protectionmodule 4. The adjustment signal Sn2 output from the second comparator512 determines the first field-effect transistor 424 to turn on or off.Vref1>Vref2.

In the embodiment, the LED string 21 comprises at least one LED 211.

The following is a detailed description of the working process for theLED backlight driving circuit in FIG. 4. The regulating voltage of thefirst voltage regulator 421 is V1, that of the second voltage regulatoris V2, that of the third voltage regulator 423 is V3, the initialpredetermined OVP voltage is V1+V2+V3, and the OVP voltage changes withpractical operation voltage of the LED string 21 after the LED string 21works steadily.

(a) When the operation voltage of the LED string 21 is at normal range,which means the voltage at the positive terminal of the LED string 21 isnormal, the relation among the dividing voltage Vfb of the dividercircuit 52, the reference voltages Vref1 and Vref2 is Vref2<Vfb<Vref1.At the time, both the adjustment signal Sn1 from the first comparator511 and the adjustment signal Sn2 from the second comparator 512 are atlow level, and the OVP voltage set by the overvoltage protection module4 is V1+V2 when the first field-effect transistor 424 and the secondfield-effect transistor 425 turn off and the third field-effecttransistor 426 turns on.

(b) When the operation voltage of the LED string 21 is out of thestandard value, which means the voltage at the positive terminal of theLED string 21 is over, the relation among the dividing voltage Vfb ofthe divider circuit 52, the reference voltages Vref1 and Vref2 isVfb>Vref1>Vref2. At the time, the adjustment signal Sn1 from the firstcomparator 511 is at high level, the adjustment signal Sn2 from thesecond comparator 512 is at low level, and the OVP voltage set by theovervoltage protection module 4 is V1+V2+V3 to raise the OVP voltagecorrespondently when the first field-effect transistor 424 and the thirdfield-effect transistor 426 turn off and the second field-effecttransistor 425 turns on. Since the OVP voltage arises, the situationthat the LED string fails to lit due to higher operation voltage of LEDstring but lower OVP standard value is avoided.

(c) When the operation voltage of the LED string 21 is under thestandard value, which means the voltage at the positive terminal of theLED string 21 is lower, the relation among the dividing voltage Vfb ofthe divider circuit 52, the reference voltages Vref1 and Vref2 isVfb>Vref1>Vref2. At the time, the adjustment signal Sn1 from the firstcomparator 511 is at low level, the adjustment signal Sn2 from thesecond comparator 512 is at high level, and the OVP voltage set by theovervoltage protection module 4 is V1 to lessen the OVP voltagecorrespondently when the first field-effect transistor 424 and the thirdfield-effect transistor 426 turn on and the second field-effecttransistor 425 turns off. It turns out to prevent component from damageattributed by taking a long time to boost a voltage of a positive end ofthe LED to the OVP voltage because of lower practical operation voltagein the LED string with over OVP voltage.

There is no current through the fourth resistor 412 if the voltage ofthe positive terminal of the LED string is under the OVP voltage, thenthe fourth field-effect transistor 412 turns off, the control signal ENis at high level, and the control chip U1 runs normally. On thecontrary, there is current through the fourth resistor 412 if thevoltage of the positive terminal of the LED string is over the OVPvoltage, then the fourth field-effect transistor 412 turns on, thecontrol signal EN is at low level, the control chip U1 stops workingnormally, and it turns out to achieve overvoltage protection.

In sum, the present invention provides the overvoltage protectioncircuit capable of automatically adjusting an overvoltage according toan operation voltage of a load so that it prevents components fromdamage attributed by errors or tardy protection on account of tremendousvoltage change of the load. Specifically, the LED backlight drivingcircuit, comprising the overvoltage protection circuit, is capable ofmonitoring the operation voltage in the LED string and controlling theOVP voltage value based on the operation voltage. The overvoltageprotection module decreases the OVP voltage value through the firstadjustment signal from the overvoltage adjusting module if the operationvoltage in the LED string is under standard value. The overvoltageprotection module increases the OVP voltage value through the secondadjustment signal from the overvoltage adjusting module if the operationvoltage in the LED string is over standard. In this way, itautomatically adjusts the OVP voltage value based on the practicaloperation voltage in the LED string if there is a tremendous voltagechange in the LED string. It effectively prevents from thenon-lightening error attributed by over practical operation voltage inthe LED string with lower OVP voltage, or prevents components fromdamage attributed by taking a long time to boost a voltage of a positiveend of the LED to the OVP voltage because of lower practical operationvoltage in the LED string with over OVP voltage.

In addition, terms, such as first, second, A, B, (a), (b) or the likemay be used herein when describing components of the present invention.Each of these terminologies is not used to define an essence, order orsequence of a corresponding component but used merely to distinguish thecorresponding component from other component(s). It should be noted thatif it is described in the specification that one component is“connected,” “coupled” or “joined” to another component, a thirdcomponent may be “connected,” “coupled,” and “joined” between the firstand second components, although the first component may be directlyconnected, coupled or joined to the second component.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

What is claimed is:
 1. An overvoltage protection circuit, comprising: avoltage boost circuit, for boosting an input voltage into an outputvoltage in need and providing the output voltage to a load; a voltagecontrol module, for controlling the voltage boost circuit to boost aninput voltage into an output voltage in need and for providing theoutput voltage to the load and to drive the load in a constant current;an overvoltage protection module, for monitoring a voltage of a positiveterminal of the load and generating a control signal based on thevoltage of the positive terminal of the load and a predeterminedovervoltage, the control signal being used to control the voltagecontrol module to enable or disable; and an overvoltage adjustingmodule, for monitoring an operation voltage of the load to generate anadjustment signal according to the operation voltage, the adjustmentsignal being used for adjusting an overvoltage in the overvoltageprotection module.
 2. The overvoltage protection circuit of claim 1,wherein the overvoltage protection module generates a first controlsignal to control the voltage control module to enable if theovervoltage protection module monitors that the voltage of the positiveterminal of the load is under the overvoltage of the overvoltageprotection module, and the overvoltage protection module generates asecond control signal to control the voltage control module to disableif the overvoltage protection module monitors that the voltage of thepositive terminal of the load is over the overvoltage of the overvoltageprotection module, wherein if the overvoltage adjusting module monitorsthe operation voltage in the load is under a standard value, theovervoltage adjusting module generates a first adjustment signal, andthe overvoltage protection module decreases the overvoltage according tothe first adjustment signal, and if the overvoltage adjusting modulemonitors the operation voltage in the LED string is over the standardvalue, the overvoltage adjusting module generates a second adjustmentsignal, and the overvoltage protection module augments overvoltageaccording to the second adjustment signal.
 3. A light emitting diode(LED) backlight driving circuit, comprising: a voltage boost circuit,for boosting an input voltage into an output voltage in need andproviding the output voltage to an LED string; a voltage control module,for controlling the voltage boost circuit to boost an input voltage intoan output voltage in need and providing the output voltage to an LEDstring and to drive the LED string in a constant current; an overvoltageprotection module, for monitoring a voltage of a positive terminal ofthe LED string and for generating a control signal based on the voltageof the positive terminal of the LED string and a predeterminedovervoltage, the control signal being used to control the voltagecontrol module to enable or disable; and an overvoltage adjustingmodule, for monitoring an operation voltage of the LED string togenerate an adjustment signal according to the operation voltage, theadjustment signal being used for adjusting an overvoltage in theovervoltage protection module.
 4. The LED backlight driving circuit ofclaim 3, wherein the overvoltage protection module generates a firstcontrol signal to control the voltage control module to enable if theovervoltage protection module monitors that the voltage of the positiveterminal in the LED string is under the overvoltage of the overvoltageprotection module, and the overvoltage protection module generates asecond control signal to control the voltage control module to disableif the overvoltage protection module monitors that the voltage of thepositive terminal in the LED string is over the overvoltage of theovervoltage protection module.
 5. The LED backlight driving circuit ofclaim 4, wherein if the overvoltage adjusting module monitors theoperation voltage in the LED string is under a standard value, theovervoltage adjusting module generates a first adjustment signal, andthe overvoltage protection module decreases the overvoltage according tothe first adjustment signal, and if the overvoltage adjusting modulemonitors the operation voltage in the LED string is over the standardvalue, the overvoltage adjusting module generates a second adjustmentsignal, and the overvoltage protection module augments overvoltageaccording to the second adjustment signal.
 6. The LED backlight drivingcircuit of claim 5, wherein the overvoltage protection module comprisesan adjustment circuit for adjusting overvoltage according to anadjustment signal from the overvoltage adjusting module, and aprotection circuit for monitoring and comparing a voltage of thepositive terminal of the LED string with the overvoltage to generate acontrol signal coupling to the voltage control module.
 7. The LEDbacklight driving circuit of claim 6, wherein the adjustment circuitcomprises a first voltage regulator, a second voltage regulator, a thirdvoltage regulator, a first field-effect transistor, a secondfield-effect transistor, a third field-effect transistor and a thirdresistor; wherein the first voltage regulator, the second voltageregulator and the third voltage regulator are in electrical series, acathode of the first voltage regulator is coupled to the protectioncircuit, an anode of the third voltage regulator is grounded; gates ofthe first and the second field-effect transistors are respectivelycoupled to the overvoltage adjusting module, the adjustment signal fromthe overvoltage adjusting module controls the first field-effecttransistor and the second field-effect transistor to turn on or off, adrain of the first field-effect transistor is coupled to a cathode ofthe first voltage regulator, a drain of the second field-effecttransistor is coupled to a gate of the third field-effect transistor andthen coupled to a third reference voltage through the third resistor, adrain of the third field-effect transistor is coupled to an anode of thesecond voltage regulator, sources of the first, the second and the thirdfield-effect transistors are all grounded; and the protection circuitcomprises a fourth resistor and a fourth field-effect transistor, oneend of the fourth resistor is coupled to a positive end of the LEDstring, the other end of the fourth resistor is coupled to a gate of thefourth field-effect transistor and then coupled to the cathode of thefirst voltage regulator, a drain of the fourth field-effect transistoroutputs a control signal coupling to the voltage control module, and asource of the fourth field-effect transistor is grounded.
 8. The LEDbacklight driving circuit of claim 5, wherein the overvoltage adjustingmodule comprises a divider circuit, for monitoring a operation voltageof the LED string and generating a dividing voltage, and a comparisoncircuit, for generating an adjustment signal coupling to the overvoltageprotection module based on the dividing voltage.
 9. The LED backlightdriving circuit of claim 8, wherein the comparison circuit comprises afirst comparator and a second comparator, and wherein an out-of-phaseinput terminal of the first comparator receives a first referencevoltage, an in-phase input terminal of the second comparator receives asecond reference voltage, an in-phase input terminal of the firstcomparator and an out-of-phase input terminal of the second comparatorrespectively receive the dividing voltage from the divider circuit, andoutput terminals of the first and the second comparators respectivelycouple adjustment signals to the overvoltage protection module, whereinthe first reference voltage is over the second reference voltage. 10.The LED backlight driving circuit of claim 8, wherein the dividercircuit comprises a first resistor and a second resistor, wherein oneend of the first resistor is coupled to the positive terminal of the LEDstring, the other end of the first resistor is coupled to one end of thesecond resistor and then coupled to the comparison circuit, and theother end of the second resistor is grounded.
 11. The LED backlightdriving circuit of claim 9, wherein the divider circuit comprises afirst resistor and a second resistor, wherein one end of the firstresistor is coupled to the positive terminal of the LED string, theother end of the first resistor is coupled to one end of the secondresistor and then coupled to the comparison circuit, and the other endof the second resistor is grounded.
 12. A liquid crystal displaycomprising a light emitting diode (LED) backlight source driven by anLED backlight driving circuit, the LED backlight driving circuitcomprising: a voltage boost circuit, for boosting an input voltage intoan output voltage in need and providing the output voltage to an LEDstring; a voltage control module, for controlling the voltage boostcircuit to boost an input voltage into an output voltage in need andproviding the output voltage to an LED string and to drive the LEDstring in a constant current; an overvoltage protection module, formonitoring a voltage of a positive terminal of the LED string and forgenerating a control signal based on the voltage of the positiveterminal of the LED string and a predetermined overvoltage, the controlsignal being used to control the voltage control module to enable ordisable; and an overvoltage adjusting module, for monitoring anoperation voltage of the LED string to generate an adjustment signalaccording to the operation voltage, the adjustment signal being used foradjusting an overvoltage in the overvoltage protection module.
 13. Theliquid crystal display of claim 12, wherein the overvoltage protectionmodule generates a first control signal to control the voltage controlmodule to enable if the overvoltage protection module monitors that thevoltage of the positive terminal in the LED string is under theovervoltage of the overvoltage protection module, and the overvoltageprotection module generates a second control signal to control thevoltage control module to disable if the overvoltage protection modulemonitors that the voltage of the positive terminal in the LED string isover the overvoltage of the overvoltage protection module.
 14. Theliquid crystal display of claim 13, wherein if the overvoltage adjustingmodule monitors the operation voltage in the LED string is under astandard value, the overvoltage adjusting module generates a firstadjustment signal, and the overvoltage protection module decreases theovervoltage according to the first adjustment signal, and if theovervoltage adjusting module monitors the operation voltage in the LEDstring is over the standard value, the overvoltage adjusting modulegenerates a second adjustment signal, and the overvoltage protectionmodule augments overvoltage according to the second adjustment signal.15. The liquid crystal display of claim 14, wherein the overvoltageprotection module comprises an adjustment circuit for adjustingovervoltage according to an adjustment signal from the overvoltageadjusting module, and a protection circuit for monitoring and comparinga voltage of the positive terminal of the LED string with theovervoltage to generate a control signal coupling to the voltage controlmodule.
 16. The liquid crystal display of claim 15, wherein theadjustment circuit comprises a first voltage regulator, a second voltageregulator, a third voltage regulator, a first field-effect transistor, asecond field-effect transistor, a third field-effect transistor and athird resistor; wherein the first voltage regulator, the second voltageregulator and the third voltage regulator are in electrical series, acathode of the first voltage regulator is coupled to the protectioncircuit, an anode of the third voltage regulator is grounded; gates ofthe first and the second field-effect transistors are respectivelycoupled to the overvoltage adjusting module, the adjustment signal fromthe overvoltage adjusting module controls the first field-effecttransistor and the second field-effect transistor to turn on or off, adrain of the first field-effect transistor is coupled to a cathode ofthe first voltage regulator, a drain of the second field-effecttransistor is coupled to a gate of the third field-effect transistor andthen coupled to a third reference voltage through the third resistor, adrain of the third field-effect transistor is coupled to an anode of thesecond voltage regulator, sources of the first, the second and the thirdfield-effect transistors are all grounded; and the protection circuitcomprises a fourth resistor and a fourth field-effect transistor, oneend of the fourth resistor is coupled to a positive end of the LEDstring, the other end of the fourth resistor is coupled to a gate of thefourth field-effect transistor and then coupled to the cathode of thefirst voltage regulator, a drain of the fourth field-effect transistoroutputs a control signal coupling to the voltage control module, and asource of the fourth field-effect transistor is grounded.
 17. The liquidcrystal display of claim 15, wherein the overvoltage adjusting modulecomprises a divider circuit, for monitoring a operation voltage of theLED string and generating a dividing voltage, and a comparison circuit,for generating an adjustment signal coupling to the overvoltageprotection module based on the dividing voltage.
 18. The liquid crystaldisplay of claim 17, wherein the comparison circuit comprises a firstcomparator and a second comparator, and wherein an out-of-phase inputterminal of the first comparator receives a first reference voltage, anin-phase input terminal of the second comparator receives a secondreference voltage, an in-phase input terminal of the first comparatorand an out-of-phase input terminal of the second comparator respectivelyreceive the dividing voltage from the divider circuit, and outputterminals of the first and the second comparators respectively coupleadjustment signals to the overvoltage protection module, wherein thefirst reference voltage is over the second reference voltage.
 19. Theliquid crystal display of claim 17, wherein the divider circuitcomprises a first resistor and a second resistor, wherein one end of thefirst resistor is coupled to the positive terminal of the LED string,the other end of the first resistor is coupled to one end of the secondresistor and then coupled to the comparison circuit, and the other endof the second resistor is grounded.
 20. The liquid crystal display ofclaim 18, wherein the divider circuit comprises a first resistor and asecond resistor, wherein one end of the first resistor is coupled to thepositive terminal of the LED string, the other end of the first resistoris coupled to one end of the second resistor and then coupled to thecomparison circuit, and the other end of the second resistor isgrounded.